PROJECT SUMMARY Noroviruses (NV) are the leading cause of acute gastroenteritis globally, causing an estimated 267 million infections and 200,000 deaths each year. Efforts to limit NV disease have been hampered by significant antigenic diversity and frequent emergence of novel strains, making vaccine development difficult. Recently it has been appreciated that NVs cause persistent infections in asymptomatic individuals, thereby creating a reservoir for viral dissemination and immune-driven evolution. How NVs evade host immunity and persist in healthy individuals is unknown and is the focus of this application. We have used the mouse NV (MNV) system to study CD8 T cell responses to acutely-cleared (CW3) vs. persisting (CR6) strains. MNV-CR6 persisted after primary infection even after we introduced a mutation that enhanced its CD8 T cell immunogenicity (strain MNV-CR6F?Y) and resulted in large numbers of MNV-specific CD8 T cells. These MNV-specific CD8 T cells had no intrinsic defects and trafficked to the intestine, responded to ex vivo peptide stimulation, and proliferated in vivo in response to MNV-CW3, yet failed to prevent establishment of chronic MNV-CR6F?Y infection. Next we immunized mice via primary infection with MNV-CW3 and challenged these mice with MNV- CR6F?Y. Immune mice initially controlled MNV-CR6F?Y, however in most cases viral replication rebounded and chronic infection was established. Subsequent adoptive transfer experiments showed that pre-existing MNV- specific CD8 T cells initially respond to challenge with MNV-CR6F?Y, however within 72 hours become apparently ignorant of ongoing viral replication. Based in these findings we hypothesize that chronic strains of MNV evade adaptive immune responses by persisting in an immunoprivileged cellular and/or anatomic niche. We will test this hypothesis by using innovative methods and novel molecular tools to define the cellular targets of MNV during acute and chronic stages of infection. Furthermore, we intend to image infected cells within their local tissue environment to gain an understanding of key cellular interactions that determine MNV clearance vs. persistence. These experiments will provide important insights into viral persistence and immune evasion at mucosal sites and are a direct extension of the K-funded research carried out by the applicant. Moreover these experiments will be relevant to future vaccine design efforts. It is expected that this R03 award will greatly facilitate the PI's transition to independence.